1. Technical Field
This invention relates to printing with moving media dot matrix printers and, more particularly, to a method of printing font character bars comprised of a plurality of rows in a direction of printing movement perpendicular to the font character bars which are in turn comprised of a plurality of close adjacent dots to improve print quality at printing speeds which induce elongated dot smear in the direction of printing movement comprising the step of removing one dot or more from an end of each of the plurality of rows of a font character bar which is otherwise over-widened from dot smear.
2. Background Art
Moving dot matrix printing devices have long been employed as output devices for computers. The term "moving dot matrix printing device" is employed to differentiate printers such as thermal transfer and impact dot matrix from laser printers which employ a technique more akin to xerography. What is being addressed is relative movement between the media and the printhead. Either (or both) can be moving with regard to the present invention and the problem addressed and solved thereby.
A typical high-speed impact dot matrix printer is shown in simplified form in FIG. 1 where it is generally indicated as 10. The printer 10 employs a printhead 12 which is driven bi-directionally across the media 14 by a motor 16 through a tape drive 18. In slower speed printers, the motor 16 can be a stepping motor which can be used by the controlling logic 20 to both drive and accurately position the printhead 12 horizontally on the media 14. With high-speed printers as wherein the present invention is particularly useful, a position sensor 22 provides the horizontal step position of the printhead 12 to the logic 20 so that the logic 20 knows when to fire the printhead 12 to create the dots on the media 14 forming the characters being printed. The printheads 12 come in a variety of printing patterns with the print pin(s) thereof comprising everything from a single dot to multiple dots as well as straight lines, slanted lines, and cluster patterns. The present invention is most applicable to printheads having one or more print pins oriented in a straight vertical line with respect to the horizontal movement of the printhead 12.
A thermal printing printer 24 as wherein the present invention is applicable is typically configured as in FIGS. 2 and 3. In this case, the printhead 12' is stationary across the path of movement of the media 14 and the media 14 is moved under the printhead in steps by a platen roller 26 driven by a stepping motor 26 under the control of logic 20', which also controls the printing by the printhead 12'. A thermal printhead 12' is comprised of a plurality of adjacent thermal printing elements (not shown) which heat when an electrical signal is applied to them. By placing a ribbon (not shown) containing a heat-transferable ink between the printhead 12' and the media 14, dots or pixels of the ink can be selectively transferred to the surface of the media 14 to create the characters and/or graphics being output. Ideally, each thermal printing element would reach its printing temperature instantly and instantly return to its ambient temperature printing a perfect dot 28 as in FIG. 4. Unfortunately, while performance is good, it is not ideal. Rather, there is a ramp-up time, a printing time, and a ramp-down time of the temperature of the printing elements. Thus, if there is movement of the media 14 under the printhead 12', instead of the ideal dot 28 of FIG. 4, what is printed is the smeared or extended dot 28' of FIG. 5.
A similar situation can occur in the impact dot matrix printer 10. The printhead 12 moves back and forth over the media 14 while the media 14 moves up line by line under the path of the printhead 12 at the end of each printing pass of the printhead 12. Early slow impact dot matrix printers disposed an inked ribbon between the printhead and the media and were not concerned with the speed of ribbon movement relative to the printhead speed. As printhead speeds were increased to increase the printer throughput, it was found that smear would occur if there was a difference between the ribbon speed and the printhead speed. Thus, the ribbon speeds were linked to the printhead speeds. Bi-directional printing, ultra-high printhead speeds, differences in ribbon quality, and relative changes at the ends of ribbons in rebound printers, for example however, created a situation in which smear can take place once again and, therefore, is a factor which must be taken into consideration. Thus, instead of getting a clear dot 28 in the shape of the end of the printing pin as depicted in FIG. 6, an elongated dot 28' as in FIG. 7 is produced. Ribbon type, age, ink content, and printer speed all affect the degree of elongation. In both the printers 10 and 24, however, speed is the most influential factor relative to dot elongation in the direction of relative printhead movement.
A typical dot matrix font character definition is shown in FIG. 8. To print the capital "P" using dots by any printing method, the printer will print the series of dots 28 shown. Where smearing as in FIG. 5 or 7 occurs, the strokes of the character will be extended in the direction of movement as depicted in FIG. 9. Such printing is, of course, generally unacceptable.
Wherefore, it is the object of the present invention to provide a way of compensating the characters printed by a moving dot matrix printer for smear along the direction of movement caused by any source.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.